A tunable YIG (Yttrium-Iron-Garnet) oscillator is an oscillator that generates signals in the microwave band from 500 MHz up to 40 GHz. The heart of the YIG oscillator is a YIG spherical resonator. A YIG spherical resonator has a natural resonant frequency that is proportional to the strength of the magnetic field going through the YIG spherical resonator. YIG resonators resonate when a magnetic field is applied to a YIG (Y.sub.3 Fe.sub.5 O.sub.12) sphere. The output frequency of a YIG oscillator is a function of the magnetic field strength that is supplied to the YIG sphere by an associated permanent magnet.
YIG oscillators appeal to customers because they generate an output signal that is very clean. A YIG oscillator that is specified to have an output frequency of 5 GHz will output a frequency of 5 GHz with very little deviation. YIG oscillators are stable and have little jitter. They have little phase noise, which is a measure of spectral purity. An oscillator which is not spectrally clean, or which has a lot of jitter is going to have trouble if another signal is placed close to the oscillator signal. If the oscillator signal has a lot of noise, it will smear into another signal that is of importance. Noisy oscillators force designers to space other oscillators further away. In the telecommunications world, bandwidth is imperative, and there are only certain bands allocated for certain microwave radios. If an oscillator has a lot of jitter and a lot of phase noise, then the oscillator is going to interfere with another oscillator with a signal operating in a nearby band.
Oscillators have important applications in cellular telephones. As the cellular telephone community expands, the applications for YIG oscillators will continue to increase. Cellular telephones require base stations to receive, amplify, and transmit communications signals. The base station receiver is basically a radio receiver that has to have a local oscillator. This local oscillator has to be a very clean, low-phase noise, local oscillator.
Another oscillator called a dielectric resonator oscillator competes with the YIG oscillator. The dielectric resonator oscillator is also a very low phase noise oscillator. The difference between the dielectric resonator oscillator and the YIG oscillator is that the YIG oscillator is tunable which enables the frequency of the YIG oscillator to be changed.
For example, if a company is building a base station transceiver, it is allocated a certain frequency or band of frequencies. If the company buys an oscillator that has to be set at 5 GHz, the company can either buy a YIG oscillator or a dielectric resonator oscillator. If the FCC reallocates frequencies or some other change, the company can no longer use the local oscillator at 5 GHz. With the dielectric resonator oscillator, a technician would have to physically go into the radio; i.e. physically remove the dielectric resonator oscillator and purchase a totally different dielectric resonator oscillator and then make sure everything worked. So there is the cost of the technician doing the work which is expensive. Moreover, the company would have to stock all kinds of dielectric resonator oscillators that have different oscillating frequencies because the frequencies may change again.
YIG oscillators on the other hand, unlike the dielectric resonator oscillators, are tunable or frequency agile. Supplying a little current to the oscillator enables the frequency of the oscillator to shift from 4 GHz to 6 GHz or any frequency in between. Referring to the base station transceiver example, a simple software manipulation can complete the frequency transformation for the company. The software manipulation does not even necessarily have to take place at the base station. It can be accomplished remotely.
A single YIG oscillator could replace as many as 50 dielectric resonator oscillators that a customer might have to stock because the customer does not know what frequency may be required in the future.
The output frequency of a YIG oscillator is a function of the magnetic strength of the permanent magnet. To precisely set the resonant frequency of YIG oscillators, accurately specified permanent magnets are needed. However, permanent magnets of accurate specification are not readily available. For example, to buy a permanent magnet that generates a 3,000 gauss airgap field from a manufacturer, in reality what would happen is the manufacturer would supply a permanent magnet that generated a magnetic field strength anywhere from 2700 gauss to 3300 gauss. A ten-percent error in the magnetic field specification is not uncommon.
If it were possible to buy precision magnets, manufacturing the YIG oscillator would be simple. The problem is that manufacturers do not supply magnets that are sufficiently precise. The impreciseness of the magnetic field also affects the phase noise. The reason that the YIG oscillator has really excellent phase noise is due to the `quality` factor of the YIG resonator. The YIG resonator has a very high `quality` factor, or Q. When the Q is distorted or lowered it is very undesirable. Thus, a higher Q provides better phase noise in the YIG oscillator. Thus, maintaining a high Q is important.
Further, because of the way permanent magnets are manufactured, permanent magnets do not generate a homogenous magnetic field. Accordingly, such permanent magnets will have uneven magnetic fields at the YIG sphere. The uneven magnetic field affects the Q factor of the YIG sphere such that different parts of the YIG sphere oscillate at slightly different frequencies. Thus, the YIG oscillator, instead of generating a nice clean signal, generates a fuzzy signal because the YIG resonator is not oscillating at one frequency. The frequency of the YIG oscillator becomes smeared.
To make the magnetic field homogeneous, a disc of ferromagnetic material is placed on the face of the permanent magnet. The ferromagnetic material smoothes the unevenness and focuses the magnetic field. The ferromagnetic material is called a "field straightener", because the ferromagnetic material straightens the uneven magnetic field from the permanent magnet.
A beneficial attribute of the field straightener is that it not only straightens the magnetic field, but the diameter of the field straightener affects the magnetic field strength at the YIG sphere. With a permanent magnet most of the magnetic field goes through the YIG sphere; however, some of the magnetic field goes around the side and back to the opposite pole of the permanent magnet. The field straightener affects how much of the magnetic field reaches the YIG sphere and how much gets shunted away. By varying the diameter of field straighteners, the magnetic field strength can be changed. Increasing the diameter of the field straightener, lower the affects of the magnetic field on the YIG sphere.
The field straighteners precisely set the magnetic field that affects the YIG sphere. It is important that the field straightener be concentric with the permanent magnet to avoid altering the magnetic field and adversely affecting the oscillating frequency of the YIG sphere.
One of the major manufacturing challenges has been to properly align the field straightener to the permanent magnet so that the field straightener can be epoxied in the correct location to the permanent magnet. Many different fixtures are used to handle different sized field straighteners adding to the cost and complexity of manufacturing YIG oscillators.
As the number of YIG oscillators required by customers increases, it becomes more desirable to develop apparatus and manufacturing methods for making YIG oscillators more efficiently and less expensively.